Stretched polypropylene films, especially biaxially stretched ones, have found extensive use as food packaging, and fiber packaging and in other applications because of their good optical properties such as transparency and gloss, mechanical properties such tensile strength and rigidity and moisture proofness. In some applications, however, these properties are not fully satisfied. In particular, in applications which require see-through characteristics, the optical non-uniformity inherent to stretched polypropylene films is a problem and greatly limits the range of their utility.
Various attempts have therefore been made to improve the optical properties of stretched polypropylene films. For example, Japanese Patent Publication Nos. 1279/1971 and 15779/1979 show that biaxially stretched films of polypropylene containing specified alicyclic hydrocarbon resins or terpene resins have improved optical properties, especially in transparency and gloss. However, the improvement of the optical properties of the biaxially stretched films obtained by the methods disclosed in these patent documents is insufficient. It is also known that since these films contain fairly large amounts of additives such as the alicyclic hydrocarbon resins, the inherent solvent resistance of polypropylene is reduced, or fuming and roll contamination occurs remarkably during film formation.